The present invention generally relates to semiconductor memory devices, and more particularly to a semiconductor memory device such as a mask read only memory (ROM) which is capable of replacing defective memory cells.
Data written in a mask ROM is fixed during the production process when an exposure is made using a photomask or an exposure is made directly without the use of the photomask. The mask ROM is suited for use in a case where there is a need to produce a large number of ROMs which store identical fixed data such as character font data. In addition, the memory cell structure of the mask ROM is simple and the size of the memory cells can easily be reduced. For this reason, the design rule is reduced and the chip size is increased recently to further improve the integration density.
On the other hand, defective memory cells increase as the integration density becomes high. As a result, there are problems in that the yield of the mask ROM becomes poor and the mask ROM becomes expensive.
Conventionally, there are methods of replacing the defective memory cells by using the so-called redundant memory cells in place of the defective memory cells, and these methods are generally employed in a dynamic random access memory (DRAM), a static RAM (SRAM) and an erasable programmable ROM (EPROM). FIG. 1 shows an essential part of an example of the DRAM which employs such a conventional method of replacing the defective memory cells.
In FIG. 1, the DRAM comprises an address buffer 1, a redundancy discriminating circuit 2, a control circuit 3, a DRAM memory cell array 4, a defective memory cell array 4a which includes defective memory cells and corresponds to one row, for example, a redundant memory cell array 5 which corresponds to one row, a redundant memory cell array 6 which corresponds to one column, a row decoder 7, a column decoder 8, a write/read amplifier 9, and an input/output circuit 10.
The redundancy discriminating circuit 2 detects whether or not an input address coincides with a preset redundant address of the defective memory cell array 4a, and makes the control circuit 3 output a control signal S.sub.R or S.sub.C when the input address coincides with the redundant address. It will be assumed for the sake of convenience that the control circuit 3 outputs the control signal S.sub.R, that is, the defective memory cell array 4a is substituted by the redundant memory cell array 5. In this case, the write/read amplifier 9 writes data in or reads data from the redundant memory cell array 5. In other words, the defective memory cells corresponding to one row is substituted by the redundant memory cells to relieve the defective memory cells, thereby improving the yield of the DRAM.
However, this method of replacing the defective memory cells of the DRAM cannot be applied to the mask ROM because the fixed data are written into the mask ROM during the production process. In the case of the mask ROM, the fixed data cannot be written into the redundant memory cells after the defective memory cells are found. In order to overcome this problem, it is conceivable to use redundant memory cells having a PROM structure. In this conceivable case, it is possible to freely write the fixed data into the redundant memory cells having the PROM structure after the defective memory cells are found.
The yield of the mask ROM can be improved according to the above conceivable method which uses the redundant memory cells having the PROM structure. But on the other hand, the integration density of the mask ROM becomes poor for the following reason. That is, the cell area of the mask ROM versus the cell area of the PROM is generally in the order of 1:30, and the cell area of the PROM is extremely large compared to that of the mask ROM. For this reason, when word lines and bit lines are used in common between the mask ROM and the PROM and the mask ROM and the PROM coexist on the chip, the integration density of the mask ROM becomes determined by the cell area of the PROM. As a result, there are problems in that the advantageous features of the mask ROM cannot be utilized and the integration density of the mask ROM becomes poor.